One type of display system is a projection display system, where a projection lens is used to project the image to a screen. Movie theaters are one example of such display systems on a large scale. More recently, television sets and digital cinema systems have been developed that also use projection lenses. The projection lens may be for either front or rear projection, depending on whether the lens is on the viewer side of the screen or behind the screen.
Spatial light modulators (SLMs) are a type of display device that may use a projection lens. In general, SLMs are arrays of pixel-generating elements that emit or reflect light to the display screen via the projection lens. The SLM modulates light by turning the pixel-generating elements on or off.
An example of an SLM is a DMD (digital micro-mirror device). A DMD is an electromechanical device, whose pixel-generating elements form an array hundreds or thousands of tiny tilting mirrors. To permit the mirrors to tilt, each is attached to one or more hinges mounted on support posts, and spaced by means of an air gap over underlying control circuitry. The control circuitry provides electrostatic forces, which cause each mirror to selectively tilt. Incident light on the mirror array is reflected by the "on" mirrors in one direction and by the "off" mirrors in the other direction. The pattern of "on" versus "off" mirrors forms an image. In most applications, the light from the DMD is projected by a projection lens to a screen.
The size of the array of an SLM's pixel-generating elements determines the aspect ratio of the image it generates. For example, an SLM might have an array size of 1024.times.768, thereby generating images with an aspect ratio of 4:3. This 4:3 aspect ratio is consistent with NTSC television broadcast signals as well as those used for personal computer displays.
As display systems become more advanced, the variety of source data that they are capable of displaying has increased. Each type of source data may have its own format, that is, its own aspect ratio and vertical and horizontal resolution. For example, one advance is the availability of "digital cinema", in which movie films are digitized for display. Movie films are characterized by a wide aspect ratio, such as 2:1. Another advance is high definition television with wide aspect ratios, such as the 16:9 format.
For television applications, the conventional approach to displaying multiple formats with the same television set has been to use various data processing techniques, such as sampling or scaling, so that the data fits the SLM. For example, the television set might have a 4:3 SLM, and for displays having a different aspect ratio, the image is scaled so that it fills the SLM in at least one dimension. This can result in a display that does not fill the screen vertically for wide-screen displays, a display known as a "letterbox" display. U.S. patent application Ser. No. 08/333,200, entitled "A Multi-Format Television Architecture", and U.S. patent application Ser. No. 08/091,852, entitled "Method and Device for Multi-Format Television", describe methods of processing data to accommodate various display formats.